At first glance, human infectious disease seems to have very little, if anything, to do with the geographical distribution of plants and animals, the amount of rainfall an area receives, or the diversity of predators within an area. But new research led by University of Wisconsin-Madison ecologist John Orrock suggests that such ecological factors could play a key role in regulating the prevalence of disease. Moreover, he has provided some of the first evidence supporting the hypothesis that predators are important to human health.

Orrock’s study, published in the journal American Naturalist, focused on the prevalence of Sin Nombre virus in deer mice (Peromyscus maniculatus) on California’s Channel Islands. Sin Nombre virus, or SNV, causes a frequently fatal disease in humans known as hantavirus pulmonary syndrome, which was first identified in the southwestern United States in 1993.

Along with biologists Brian Allan, now at the University of Illinois at Urbana-Champaign, and Charles Drost, at the U.S. Geological Survey Southwest Biological Science Center in Arizona, Orrock sought to build upon earlier predator research conducted on the Channel Islands. “We have been working to understand the dynamics of island mouse populations and how rodent behavior changes on islands with different predator communities. While doing this work, it occurred to us that the islands offered a unique opportunity to test a few recent ideas in disease ecology,” Orrock explained.

Extending 150 miles (240 km) along the Pacific coast of southern California, the Channel Islands provide an ideal system for studying of the relationship between infectious disease and biogeographic and ecological factors. The prevalence of SNV in deer mice on several of the Channel Islands is notably high. And because each of the eight islands in the chain differs in size, local climate, and species distribution, factors most relevant to disease prevalence can be teased out from those of lesser importance.

“Above all, the island system was feasible,” Orrock said. “The data that we needed were largely already in existence. All we needed to do was compile a thorough list of rodent predators from existing sources.”

To detect correlations between SNV prevalence and biogeographic and ecological factors, Orrock and colleagues used a statistical technique known as multiple linear regression. “The technique makes it possible to examine how several variables (in this case, precipitation, island area, and rodent predator richness) each make unique contributions toward explaining the variation in SNV prevalence in island mice,” Orrock noted.

The technique is especially helpful for revealing patterns that are otherwise obscured. “For example, in our data set, both SNV prevalence and the richness of island predators are higher on larger islands,” Orrock said. “Only after the effect of area on SNV is accounted for in the multiple regression model does the unique (and negative) effect of predator richness on SNV become apparent.”

Through their regression analyses, the researchers found that the prevalence of SNV was higher on islands that were large in area, on islands that received large amounts of precipitation, and on islands where mouse predators were relatively few and far between. Because of the correlative nature of the study, the researchers were unable to tease apart the influence of specific geographical features, such as elevation and perimeter, on SNV prevalence.

San Miguel Island, one of California's eight Channel Islands. Photo credit: John Orrock

The findings demonstrate the significance of ecological pressures that control rodent and predator populations and disease spread. In Orrock’s study, two important pressures included so-called top-down and bottom-up forces. He explained that bottom-up forces come into play when a population’s size is controlled primarily by the resources used by members of that population. In contrast, a population is limited by top-down forces when predators that attack the population control the size of the population.

“This concept is easiest to envision if you think of a simple food chain with plants at the bottom, mice in the middle, and predators (e.g., island foxes) at the top,” explained Orrock. “Mice are said to be bottom-up limited when the number of mice is controlled by the amount of seeds produced (often a function of environmental variables, like precipitation). Mouse populations are said to be top-down limited when predators control mouse abundance.”

He noted that in nature, however, bottom-up and top-down forces probably operate together, at least to some degree, on a population. “Our study supports this notion by finding that precipitation and island characteristics (which are likely related to resource abundance and thus bottom-up control) and predators (top-down control) are important,” he said.

The concepts of top-down and bottom-up control are relevant to disease dynamics because, according to Orrock, “Anything that affects the number of hosts (rodents in this case) can alter the likelihood that a disease is spread among hosts and maintained in a population.”

It is suspected that relationships between the prevalence of infectious disease, biogeography, and ecology also exist in mainland areas and on landmasses much larger than the Channel Islands. But as Orrock said, “Testing the importance of top predators in mainland systems is likely to be more difficult because it is hard to know which predators are present in the system at any given time.”

He added, however, that, “Climate, small mammal community composition (which contributes to an idea from disease ecology called the “dilution effect”), and rodent population density at a location can be linked to prevalence of disease in rodents.” Considering these factors, it may be possible for scientists to eventually identify patterns of disease prevalence in local mainland areas and thereby determine the risk of transmission to humans.

The next step in gaining a better understanding of the ecological regulation of disease is to perform experimental studies that manipulate the number of top predators or the resources available to rodent populations. “In our study, we never actually manipulated predator abundance, the amount of rainfall on the islands, or island size,” Orrock pointed out. “One clear limitation of our study is that it is correlative—we took advantage of natural variation in island systems to understand patterns in disease prevalence.”

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A regular Britannica Blog feature written by the encyclopedia’s own Kara Rogers, Science Up Front goes behind the headlines to bring researchers’ stories of discovery centerstage. Begun in 2009 to highlight the ingenious work of pioneering scientists and to bring greater accuracy to science reporting, Rogers goes straight to the source, exploring the latest advances in science, from medicine to nanotechnology to conservation, through first-hand interviews with researchers. The series covers all things science, so check back regularly to see who’s up on Science Up Front.